This application is based on Japanese Patent Applications Nos. 11-267121 and 11-267131 both filed Sep. 21, 1999, and No. 11-291188 filed Oct. 13, 1999, the contents of which are incorporated hereinto by reference.
1. Field of the Invention
The present invention relates in general to a method of producing a piston for a swash plate type compressor, and more particularly to a method of producing such a piston having a hollow cylindrical head portion.
2. Discussion of the Related Art
A piston used for a swash plate type compressor has a head portion which is slidably fitted in a cylinder bore formed in a cylinder block of the compressor, and an engaging portion which slidably engages the swash plate. Since the piston is reciprocated within the cylinder bore, it is desirable to reduce the weight of the piston. A single-headed piston is usually used in a swash plate type compressor of variable capacity type wherein the angle of inclination of a swash plate is variable to change the discharge capacity of the compressor. When the single-headed piston is used in the variable capacity type swash plate compressor, it is particularly required to reduce its weight in order to achieve a stable operation of the compressor and reduce the noise of the compressor during its operation. To this end, various methods have been proposed to form the piston with a hollow head portion.
As one example of the method for forming the piston with a hollow head portion, there are formed by forging a hollow cylindrical head member which is open at one of its opposite ends, and an engaging member with which a cover is integrally formed for closing the open end of the head member. The engaging member engages a swash plate of the compressor. The head member and the engaging member are connected together by welding, with the open end of the head member being closed by the cover. According to this method, the weight of the piston can be reduced by reducing the cylindrical wall thickness of the head member. In the piston constructed as described above, however, a connection, at which the head member and the engaging member are connected, is located at an axially intermediate portion of the piston, so that the piston tends to be subjected to a relatively large force at the connection during the reciprocating movement of the piston, undesirably deteriorating the durability of the piston. Further, this method pushes up the cost of manufacture of the piston.
As another method for forming the piston with a hollow head portion, a body member is formed by die-casting. The body member includes a hollow cylindrical head section which has an open end and a closed end, and an engaging section which is formed integrally with the head section. A closing member is fixed to the head section so as to close the open end. However, the head section of the body member which is formed by a conventional die-casting method does not have a sufficiently reduced cylindrical wall thickness, and needs to be subjected to a machining operation on its inner circumferential surface for reducing the cylindrical wall thickness.
The present invention was made in the light of the background art described above. It is an object of the present invention to provide a method of producing a swash plate type compressor piston whose head portion has a sufficiently reduced wall thickness and which can be produced at a relatively low cost.
The object indicated above may be achieved according to any one of the following forms or modes of the present invention, each of which is numbered like the appended claims and depend from the other form or forms, where appropriate, to indicate and clarify possible combinations of technical features of the present invention, for easier understanding of the invention. It is to be understood that the present invention is not limited to the technical features and their combinations described below. It is also to be understood that any technical feature described below in combination with other technical features may be a subject matter of the present invention, independently of those other technical features.
(1) A method of producing a piston for a swash plate type compressor including a head portion which is slidably fitted in a cylinder bore of the compressor, and an engaging section which engages a swash plate of the compressor, the method comprising the steps of forming a blank by a pore-free die-casting method, the blank including a hollow cylindrical head section which corresponds to the head portion and which has a closed end and an open end, and an engaging section which corresponds to the engaging portion and which is formed integrally with the hollow cylindrical head section at the closed end; and fixing a closing member to the hollow cylindrical head section so as to close the open end, for thereby providing the head portion of the piston.
The pore-free die-casting method prevents a gas from being trapped in a die-cast article, by introducing a molten metal such as a molten aluminum alloy into a mold cavity of a casting mold, while the mold cavity is filled with a reactive gas such as an oxygen, so that the mold cavity is placed in a highly vacuum state owing to a reaction between the molten metal and the reactive gas. The die-cast article formed by the pore-free die-casting method described above exhibits a high degree of mechanical strength with a relatively small wall thickness. The present arrangement wherein the closing member closes the open end of the hollow cylindrical head section on the side remote from the engaging section assures a higher degree of durability of the piston during use, than an arrangement wherein the closing member closes the open end of the head section on the side of the engaging section.
While the method according to the present invention is suitable for producing a single-headed piston used for a swash plate type compressor of variable capacity type, the present method is equally applicable for producing a piston used for a swash plate type compressor of fixed capacity type, and a double-headed piston.
(2) A method according to the above mode (1), further comprising a step of: preparing a casting mold consisting of two mold halves which define a parting plane that includes a centerline of the hollow cylindrical head section, at least in a portion of the mold halves which gives the head section, the casting mold being provided with a slide core which is movable in a direction parallel to the centerline of the head section and which gives an inner circumferential surface of the head section.
The arrangement according to the above mode (2) permits easy die-casting of the blank including the hollow cylindrical head section which gives the hollow head portion of the piston.
(3) A method according to the above mode (1) or (2), the hollow cylindrical head section has a cylindrical wall thickness of not larger than 1.8 mm.
The pore-free die-casting method described above is advantageous for producing a thin-walled die-cast article. By suitably determining the die-casting condition in producing the blank for the piston, the cylindrical wall thickness of the head section of the blank can be reduced to not greater than 1.8 mm, 1.5 mm, or 1.2 mm.
(4) A method according to any one of the above modes (1)xcx9c(3), the step of forming the blank comprises forming two body members each having the engaging section and the hollow cylindrical head section, the two body members being integrally connected to each other at their ends on the side of the engaging sections, such that the hollow cylindrical head sections of the two body members are concentric with each other, and such that each of the head sections of the two body members is open at one of its opposite ends which is remote from the engaging sections.
The arrangement according to the above mode (4) is effective to reduce the cost of die-casting the body member for each piston while facilitating the machining operation to be effected thereon, resulting in a reduced cost of manufacture of the piston.
(5) A method according to any one of the above modes (1)-(4), at least a portion of the inner circumferential surface of the hollow cylindrical head section is not subjected to a machining operation.
The pore-free die-casting method permits production of a thin-walled die-cast article having high degrees of mechanical strength and dimensional accuracy. Accordingly, the body member formed by the pore-free die-casting method need not be subjected to a machining operation at least on the inner circumferential surface of the cylindrical head section except its axial open end portion to which the closing member is fixed.
(6) A method according to any one of the above modes (1)xcx9c(5), wherein the step of forming a blank comprises forming by pore-free die-casing an integral precursor including a first portion which gives the blank, and a second portion which gives the closing member, by using a casting mold having a mold cavity which includes a first cavity portion for forming the first portion of the integral precursor and a second cavity portion for forming the second portion of said integral precursor, the first and second cavity portions being held in communication with each other.
In the present arrangement wherein the second portion of the integral precursor, which gives the closing member, is die-cast concurrently and together with the first portion of the integral precursor, which gives the blank. The blank and the closing member are obtained by cutting the integral precursor which can be produced in one step with high efficiency. Accordingly, the cost of manufacture of the piston by die-casting can be significantly reduced.
(7) A method according to the above mode (6), wherein the second cavity portion is held in communication with a part of the first cavity portion, which part corresponds to the engaging section of the blank.
The engaging section which gives the engaging portion of the piston has the largest wall thickness in the piston, so as to meet the structural and functional requirements of the piston. The second cavity portion for providing the closing member is held in communication with a portion of the first cavity portion which has a comparatively large dimension corresponding to the thick-walled engaging section of the piston. Accordingly, the molten metal can easily flow from the first cavity portion corresponding to the engaging portion, into the second cavity portion corresponding to the closing member, whereby the second portion of the integral precursor can be formed with high stability, so as to die-cast the closing member with high stability.